In recent years, attention is given to power storage systems for small-size, high-energy-density applications such as information and communication equipment such as personal computers, video cameras, digital still cameras and mobile phones and for large-size power applications such as electric vehicles, hybrid vehicles, auxiliary power sources of fuel cell vehicles and electricity storage devices. As one such type of power storage system, non-aqueous electrolyte batteries including lithium ion batteries, lithium batteries and lithium ion capacitors have extensively been developed.
Many kinds of non-aqueous electrolyte batteries are already in practical use, but do not have satisfactory durability for various applications. There is a problem in the long-term use of the non-aqueous electrolyte batteries under high-temperature conditions for e.g. vehicle applications because the non-aqueous electrolyte batteries largely deteriorate in performance, in particular, at temperatures of 45° C. or higher. On the other hand, there is a need for the non-aqueous electrolyte batteries to operate without troubles even under low-temperature conditions such as e.g. in cold climates for vehicle applications and electricity storage applications. It is important to secure both of high-temperature performance and low-temperature performance of the non-aqueous electrolyte batteries.
The non-aqueous electrolyte battery generally utilizes, as an ion conductor, a non-aqueous electrolyte or a non-aqueous electrolyte quasi-solidified by a gelling agent. The non-aqueous electrolyte contains a single kind of aprotic solvent, or a mixed solvent of two or more kinds of aprotic solvents, selected from ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate and the like and a lithium salt such as LiPF6, LiBF4, (CF3SO2)2NLi or (C2F5SO2)2NLi as a solute.
Conventionally, the optimization of various battery components has been studied as techniques for improving the cycle characteristics, high-temperature storage stability and durability of the non-aqueous electrolyte batteries. Non-aqueous electrolyte-related technologies are not an exception to such battery performance improvement technologies. There have been made proposals to use various additives for the purpose of preventing battery performance deteriorations caused by decomposition of electrolytes on active positive or negative electrode surfaces.
For example, Japanese Laid-Open Patent Publication No. 2000-123867 (Patent Document 1) proposes a technique for improving the performance of the battery by the addition of vinylene carbonate to the electrolyte. In this proposed technique, the electrode is coated with a polymer film by polymerization of the vinylene carbonate so as to prevent decomposition of the electrolyte at the electrode surface. However, the electrolyte increases in internal resistance due to the difficulty for lithium ions to pass through the polymer coating film and cannot provide a sufficient battery capacity at low temperatures of 0° C. or lower.
Japanese Laid-Open Patent Publication No. 2007-165125 (Patent Document 2) proposes a technique for improving the high-temperature cycle characteristics and output characteristics of the battery due to the formation of a coating film on the electrode interface by the addition of difluorobis(oxalato)phosphate and monofluorophosphate or difluorophosphate to the electrolyte. The effects of this proposed technique are however not yet sufficient. In addition, the thus-obtained battery does not show sufficient performance at low temperatures of 0° C. or lower.